1. Field of the Invention
The present invention relates to the field of couplers which are used to capture a portion of a signal conveyed by a transmission line for, in particular, measurement or control purposes. The present invention more specifically relates to the field of radiofrequency couplers between a transmit amplifier and an antenna, especially applied to mobile telephony.
2. Discussion of the Related Art
FIG. 1 very schematically illustrates the general structure of a distributed coupler 1, that is, with transmission lines of the type to which the present invention applies, as opposed to couplers with localized inductive and capacitive elements.
Coupler 1 is interposed between an amplifier 2 (PA) for amplifying a signal Tx to be transmitted, and a transmit antenna 3. The function of coupler 1 is to extract, between terminals CPLD and ISO of a secondary line 12, a signal proportional to the signal transiting over a main transmission line 11, that is, between terminals IN and DIR, respectively connected to the output of amplifier 2 and to the input of antenna 3.
Signal G extracted by coupler 1 is exploited by a circuit 4 (DET), for example to control the power of amplifier 2 or to turn it off in case of a need for protection, for example, in case of a disappearing of antenna 3.
This is an example of application to mobile telephony where the highest power consumption is due to the transmission chain and where the circuit power consumption is generally desired to be minimized. In receive mode, a mobile phone exploits a low-noise amplifier (LNA), the gain of which is generally fixed and for which a coupler is accordingly not necessary.
The coupler of FIG. 1 is a bidirectional coupler in that it detects a signal present on transmission line 11 in both directions: a forward signal (FWD) transiting from IN to DIR will be coupled towards output CPLD, and a reverse signal (REV) transiting from DIR to IN will be coupled towards output ISO. In practice, the voltages present on terminals CPLD and ISO are rectified to generate gain correction signal G.
A distributed coupler of the type shown in FIG. 1 is characterized by its coupling and its directivity. The coupling characterizes the difference between the amplitude of the main signal circulating on line 11 and the amplitude of the signal sampled from line 12. The directivity characterizes the difference between the amplitude of signal FWD, which translates as a signal coming out of terminal CPLD, and the amplitude of signal REV circulating from DIR to IN, which translates as a signal coming out of terminal ISO. The greater the amplitude difference between terminals CPLD and ISO, the greater the coupler directivity and the easier it is to detect a possible problem of antenna 3 translating as a reflection of the signal carried by line 11. Indeed, in case of a problem on the antenna (for example a disappearing thereof), the power that cannot come out is reflected, which results in an increase in the signal on terminal ISO. By detecting the potential of terminal ISO with respect to a threshold, a problem can be detected on the antenna and the transmit amplifier can then be cut off to avoid damaging it, since said amplifier generally cannot stand receiving a reflected power.
In an ideal coupler and in normal operation, the amplitude maximum of the coupled line would be present on terminal CPLD and a zero voltage would be present on terminal ISO. However, in practice, the voltage of terminal ISO is not zero, but it is generally attenuated by on the order of −30 dB with respect to the voltage of terminal DIR.
Further, a low coupling is generally searched to avoid sampling too large a portion of the output for the detection. Generally, terminal CPLD reproduces a signal attenuated by on the order of from −15 to −20 dB with respect to the signal transiting from terminal IN to terminal DIR.
Accordingly, the directivity of a conventional coupler is on the order of from −10 dB to −15 dB (−30−(−20) to −30−(−15)).
Now, especially to ease the detection of a problem on the antenna, a higher directivity is desired.
To improve the directivity, the coupler can be enlarged by making conductive sections 11 and 12 close to a length of λ/4, where λ represents the wavelength corresponding to the central frequency of the desired coupler passband. However, developing a distributed coupler at a λ/4 length results in a very bulky coupler and increases insertion losses.
FIG. 2 shows a conventional embodiment of a coupler 10 with an improved directivity. This coupler of distributed type comprises two conductive lines 11 and 12 and two capacitors Cp respectively connecting terminals iN and CPLD and terminals DIR and ISO. Such capacitors enable increasing the coupler directivity by drawing the values of the line impedances closer to one another. However, a disadvantage of such a solution is that at frequencies of several hundreds of MHz, the capacitance values are very small (on the order of one femtofarad). In practice, such values make the implementation almost impossible since the values of capacitances Cp come close to the values of stray capacitances which can then not be neglected. Now, the features of the coupler significantly degrade as soon as it is departed from the values selected, according to the coupler passband, for capacitors Cp.
Examples of couplers of the type described in relation with FIG. 2 are described in U.S. Pat. No. 4,937,541 and in German patent application 19749912, both of which are incorporated herein by reference.